1. Field of the Invention
The present invention relates to a combustion process and an apparatus therefor containing separate injectors to introduce separately a fuel and an oxidant in the combustion chamber of a furnace, so that the fuel burns with the oxidant in a wide luminous flame, and whereby the combustion of the fuel with the oxidant generates reduced quantities of nitrogen oxides (NO.sub.x).
2. Related Art
Industrial high temperature processes, such as glass or frit melting, ferrous and non ferrous materials smelting, use large amounts of energy to transform a variety of raw materials into a hot molten product, that is then casted, formed or otherwise disposed of in further stages of the industrial process. This operation is generally performed in large furnaces, that can produce as much as 500 tons per day of molten material. Combustion in the furnace of a fossil fuel, such as natural gas, atomized fuel oil, propane, or the like, with an oxidant that contains oxygen is a preferred method of supplying the energy. In some cases, the combustion is supplemented by electric heating. Most of the time, the fuel and the oxidant are introduced in the furnace through burners, in order to generate flames. The transfer of energy from the flames to the material to be melted results from the combination of convection at the surface of the material, and radiation to the surface or into the material if it is transparent to the radiation. Flames that are highly radiative (usually referred to as luminous flames), are usually preferred, because they provide better heat transfer and, thus, higher fuel efficiency.
For flame heating, it is also very important to have the energy from the flame evenly distributed above the surface of the material to be melted. Otherwise, hot and cold regions may co-exist in the furnace, which is not desirable. The quality of products manufactured with material melted in such a furnace is often poor. For example, in a bath of molten glass, there may be glass stones in cold regions, and accelerated volatilization of glass in hot regions. Also, broad flames are preferred because they yield a better bath coverage.
In many countries, particularly the United States, increasingly stringent regulations are being promulgated regarding emissions of NO.sub.x. It is, therefore, important to develop combustion techniques wherein NO.sub.x formation is limited. In very high temperature processes, NO.sub.x formation is promoted by long residence times of oxygen and nitrogen molecules in hot regions of the flame and the furnace. The use of substantially pure oxygen (about 90% O.sub.2 or higher) instead of air as the oxidant has proven to be very successful in reducing the NO.sub.x emissions by as much as 90%, since all nitrogen is eliminated. However, substitution of air by substantially pure oxygen increases the flame temperature, and thus creates regions in the furnace where the reactivity of nitrogen with oxygen is high, and wherein the formation of NO.sub.x may proportionally increase, even though it is globally decreased when compared to combustion with air. Also, it is impossible in practice to eliminate all nitrogen from a furnace, because industrial furnaces are not tight to air leaks, the fuel usually contains some nitrogen, and oxygen supplied from non-cryogenic sources, such as oxygen produced by a Vacuum Swing Adsorption plant (VSA), contains a small residual nitrogen concentration.
Conventional methods of combusting fuel and oxygen for heating furnaces utilize post mix oxy-fuel burners. Conventional oxy-fuel burners have a metallic body with inlets for a fuel and an oxidant with a high concentration of molecular oxygen, and means to transport the streams with separate coaxially oriented channels to multiple injectors located at the burner tip. These burners generate high temperature flames with the shape of a narrow pencil at the burner tip, which needs to be located far enough into the furnace, to avoid or reduce overheating of the furnace walls. As a consequence of the high temperatures encountered in melting furnaces, one important drawback of these burners is the need for cooling, usually a jacket where a circulating fluid such as water provides the cooling. Such a burner is described, for example, in British Patent 1,215,925. Severe corrosion problems for the cooling jacket can arise particularly when the furnace atmosphere contains condensable vapors.
The gas cooled oxy-fuel burner is an improvement of the water-cooled burner. The body of the burner is protected from the furnace radiation by a refractory brick often referred to as a burner block, that possesses a substantially cylindrical cavity that opens into the furnace. The burner is usually mounted at the back of the cavity, and it usually contains concentric injectors of fuel and oxidant located in the cavity, recessed from the furnace inner wall. The brick and the burner are cooled by a peripheral annular flow of gas, usually the oxidant gas. Such burners are described e.g. in U.S. Pat. No. 5,346,390 and U.S. Pat. No. 5,267,850. With this type of burner, combustion starts in the burner block before reaching the furnace. Thus, the flame is confined in and directed by the cylindrical cavity as a narrow axisymmetric jet, and provides insufficient covering of the melt in the furnace. These flames have high peak temperatures and generate relatively large amounts of NO.sub.x, because there is a direct contact between the oxygen and the fuel without dilution by the combustion products.
Another drawback of these gas cooled burners is that the flame may overheat and damage the furnace refractory wall because it starts in the wall itself. Also recirculation zones under the flame itself tend to accelerate refractory wear when the furnace atmosphere chemically reacts with the refractory material of the furnace wall which may reduce the furnace lifetime.
British Patent 1,074,826 and U.S. Pat. No. 5,299,929 disclose burners containing alternated multiple oxygen and fuel injectors in parallel rows in order to obtain a flatter flame. Although this brings an improvement in terms of coverage of the melt, these burners still produce relatively large amounts of NO.sub.x. Another drawback of these burners is that they are mechanically complex to build in order to obtain a flat flame.
It is also known to inject fuel and oxidant streams by separate, distinct injectors into a combustion chamber to generate flames detached from the furnace wall, with the aim of reducing refractory wear. One such apparatus is described in U.S. Pat. No. 5,302,112 wherein fuel and oxidant jets are injected at a converging angle into a furnace, which yields good mixing of the oxidant and fuel gases at the converging point of the two jets, thus enhancing the combustion rate but shortening the flame. However, the flame of such a burner has a high peak temperature and large quantities of nitrogen oxides are created in the furnace. To decrease this high peak temperature and significantly reduce formation of NO.sub.x it has been suggested in U.S. Pat. No. 4,378,205 to inject the fuel and/or the oxidant jets at very high velocities and to use separate injections of fuel and oxidant gases wherein the fuel and/or the oxidant jets entrain combustion products contained in the furnace atmosphere, and are diluted before the actual combustion between the fuel and the oxidant. However, the flames generated by these burners are almost invisible, as disclosed therein, col. 9, lines 58-65. It is, thus, extremely difficult for a furnace operator to determine and/or control the location of the combustion zones, and whether or not the burner apparatus is actually turned on, which may be hazardous. Another drawback of this burner is that the entrainment of combustion products promotes strong recirculation streams of gases in the furnace, which in turn accelerates the wear of the refractory walls of the furnace. Also, the use of high velocity oxidant jets requires the use of a high pressure oxidant supply, which means that the oxidant gas needs to be either produced or delivered at high pressure (the fuel gas is usually at relatively high pressure) or that the oxidant gas, such as the low pressure oxygen gas usually supplied by a VSA unit, has to be recompressed before being injected into the furnace.
Burners in use today typically are only designed to use gaseous fuel or liquid fuels (perhaps by spray of the liquid fuel), but cannot burn both types of fuel simultaneously, or switch easily from gaseous fuel to liquid fuel.
Liquid fuels present their own problems to the combustion artisan. The liquid fuel is typically atomized, and there are several different techniques available for the atomization of liquid fluids. The object is to produce jets of liquid fluid droplets (also called "spray") which have defined geometric characteristics. The usual liquid fuels are not particularly flammable in the liquid state: only in the gaseous state are they able to support an oxidation reaction sufficiently rapid to give rise to the appearance of a flame. When one wishes to obtain stable flames with fuels that are liquid or viscous at ambient temperature, the principal difficulty is thus to "shrewdly condition" this liquid in such a way that it evaporates rapidly in order to support oxidation reactions in the interior of the flame.
The method currently used to achieve this "shrewd conditioning" consists of atomizing the fuel in the form of droplets: thus, for a given quantity of fuel, this makes possible a substantial increase in the amount of liquid surface exposed to the oxidant (the smaller the drops are, the greater will be the interfacial surface--the site of evaporation).
In simplified terms there are three major methods for achieving atomization of a liquid:
1. rotating cup atomization involves shredding the fluid with the air of a moving mechanical element. PA1 2. in mechanical atomization the fuel is compressed to very high pressures (15 to 30 bars), thus imparting to it a high kinetic energy. This energy results in shearing of the liquid when it is brought into contact with the exterior atmosphere and thus results in the formation of droplets. PA1 3. gaseous-fluid-assisted atomization can be used to arrive at a similar result while achieving a saving on high pressures (2 to 6 bars). PA1 a refractory block adapted to be in fluid connection with sources of oxidant and fuel, the refractory block having a fuel and oxidant entrance end and a fuel and oxidant exit end, the exit end having fuel exits and oxidant exits, the refractory block further having a plurality of fuel cavities, at least two of the fuel cavities defining a first fuel plane, and a plurality of oxidant cavities defining a second oxidant plane, the fuel cavities being more numerous than the oxidant cavities. PA1 a) at least two fuel injectors defining a first plane; PA1 b) at least one oxidant injector; PA1 c) a wall through which the oxidant and the fuel injectors protrude into a combustion chamber, the injectors removably secured in the wall; PA1 a) providing a supply of an oxidant fluid stream; PA1 b) injecting the oxidant fluid stream into a combustion chamber to create at least one injected oxidant fluid stream; PA1 c) providing a supply of a fuel fluid stream; PA1 d) injecting the fuel fluid stream into the combustion chamber to create at least two injected fuel fluid streams; PA1 e) creating a substantially planar sheet of fuel fluid in the combustion chamber by injecting the at least two injected fuel fluid streams into the combustion chamber, at least two of the injected fuel fluid streams being substantially located in a first fuel plane; PA1 f) intersecting the oxidant fluid stream with the sheet of fuel fluid in the combustion chamber; and PA1 g) combusting the fuel fluid with the oxidant fluid in the combustion chamber.
In simplified terms one can distinguish two types of gaseous-fluid-assisted atomization according to whether the liquid fuel and atomizing fluid are brought into contact inside or outside the atomizer head. These types may be referred to as internal atomization and external atomization.
Internal atomization is characterized by confinement of the liquid fuel and atomizing fluid in an emulsion chamber. The mode of introduction of the two fluids into this chamber can vary considerably and has a direct influence on the characteristics of the emulsion that exits from the chamber. Likewise, the internal geometry of this chamber (overall volume, vanes for producing rotation, number and diameters of the inlet and outlet orifices, and so forth) also affects the specific characteristics of the fuel/atomizing fluid mixture to be burned.
This mode of atomization generally affords an excellent quality of atomization, that is, an emulsion composed of very small particles with a very narrow particle size distribution about these small diameters. At a given fuel delivery rate, this emulsion quality is naturally a function of the atomizing fluid delivery rate employed and the pressure level prevailing in the interior of the atomizing chamber.
For external atomization, where contact between the two phases takes place outside of any confining enclosure, the emulsion is created mainly by shearing of the jet of liquid fuel by the atomizing fluid. The geometry of the outlets for the two fluids completely determines the quality of the atomization, and particle size analysis of the drops resulting from the contact shows a relatively wide diameter distribution (simultaneous-presence of small and large particles).
In the field of liquid fuel atomization, the principal known priority for the invention is published European Patent Application No. 0687858 A1, which claims an external atomization device that produces a very narrow spray angle (less than 15.degree.). This published application in particular claims that to successfully achieve this specific characteristic the angle formed between the atomizing fluid and the liquid fuel must be between 5.degree. and 30.degree..
Another disclosed liquid fuel atomization device is the one disclosed in European Patent Application No. 0335728 A2, which claims a device for the introduction of a fluid into a combustion enclosure through the expedient of several distinct conduits branching from a common main conduit.
A need exists for a burner which may operate at low pressure, particularly for the oxidant gas, while producing a wide, flat luminous flame with reduced NO.sub.x emissions, and which affords a manner of controlling flame length so as to adapt the flame to the furnace in which it is used. There also exists a need in the art for a burner having the capability of burning gaseous fuels and liquid fuels, either at the same time or in the alternative. There is a need in the combustion art for a liquid fuel atomizer that falls within the scope of the third mode of atomization, a complete device that makes possible a controlled fluid introduction into the combustion zone that is a two-phase mixture of atomizing gas and droplets of liquid fuel, wherein atomization takes place outside of the nozzle (external atomization) and yet permits forming distinct spray jets having high relative angles (5.degree. to 30.degree.). In particular the combustion art is desirous of a device for atomization of a liquid fuel using a gaseous fluid and the application of this device to a burner such as the burner assemblies described herein.